In a scanning microscope, an image can be obtained by scanning a charged particle beam or a light beam and detecting signals secondarily obtained there and then performing brightness modulation of a cathode-ray tube scanned in synchronism with the scanned charged particle beam or the scanned light beam. Intensity of the charged particle beam or the light beam to be irradiated on a sample is adjusted by taking effects on the sample such as temperature rising, contamination, charging-up into consideration. When a scanning electron microscope is taken as an example, adjusting of the intensity has been performed by changing the conditions of the electron optical system forming an electron beam to be scanned such as quantity of electrons emitted from an electron source, condensing ratio, magnitude of aperture. However, if the conditions of the electron optical system is changed, the optical axis of the electron optical system will not stay in adjustment, i.e., will be shifted. Therefore, it has been required to adjust the optical axis again.
As described above, it is required to readjust the optical axis if the intensity of electron beam to be irradiated on the sample is changed. A skilled technique is required to rapidly perform the readjustment, and it requires a 10- to 20-minute adjusting time. For this reason, an apparatus practically used is apt to be used while the intensity of the electron beam is fixed constant, and is not always used under the optimum observation condition. Therefore, there have arisen problems such as change in shape due to temperature rise, elimination of very fine portions due to contamination, deformation of image due to charging-up and so on.